The present invention relates to compositions for enhancing the bonding of silicones to substrates. More particularly, it relates to self-bonding, one package, room temperature vulcanizable, silicone compositions curable in the presence of moisture which contain a small, effective, adhesion promoting amount of a composition comprising a diorganocyclopolysiloxane.
Room temperature vulcanizing silicone compositions are presently widely known. For example, Nietzche and Wick in U.S. Pat. No. 3,065,194, disclose a family of silicone rubber compositions comprising an essentially anhydrous mixture of (1) a linear organosiloxane polymer having hydroxyl terminal groups, (2) a polyfunctional organosilicone crosslinking agent, and (3) a metal salt, chelate, organometallic compound, acid or base, which serves as a crosslinking catalyst. These compositions vulcanize or cure to rubbery solids when exposed to moisture. The compositions are extremely useful because they can be maintained for extended periods of time in a single container ready for use, e.g., sealed in caulking tubes. The user may then apply the material to a substrate and simply bring it into contact with water or water vapor to cure it. The compositions are useful as sealants, electrical insulation, coatings, dental cement, caulking compounds, expansion joints, gaskets, shock absorbers, adhesives and in many other applications.
Other one package room temperature vulcanizing silicone compositions are described in Bruner, U.S. Pat. No. 3,035,016 and Ceyzeriat, U.S. Pat. No. 3,133,891, which deal with one package compositions comprising the reaction products of acyloxy-substituted silanes and hydroxylated siloxanes. The compositions cure with the liberation of acid fragments and curing may be promoted with various agents, such as organic derivatives of tin. Also of interest is U.S. Pat. No. 3,164,614, to Brown et al., which discloses compositions of a pre-treated silanol end-stopped diorganopolysiloxane and crosslinker in combination with a crosslinking catalyst. Cooper, in U.S. Pat. No. 3,383,355, describes the preparation of an alkoxy-terminated linear siloxane polymer using a neutral, finely-divided solid catalyst, such as fuller's earth. A moisture curable RTV composition which includes a hydrocarbonoxy end-blocked diorganopolysiloxane and a metal-containing curing catalyst together with boron nitride is described in U.S. Pat. No. 3,499,859.
Still another composition is described by Cooper et al. in U.S. Pat. No. 3,542,901 which comprises a mixture of a linear siloxane having di- or tri-functional end-blocking units with a linear siloxane having chemically non-functional inert end-blocking units on one end and di- or tri-functional end-blocking units on the other end, and further including a catalyst and crosslinker. Additionally of interest are: Brown et al., U.S. Pat. No. 3,122,522, who combine organopolysiloxane intermediates containing condensable "cellosolvoxy" groups with a catalyst; Brown et al., U.S. Pat. No. 3,170,894, who combine organopolysiloxane intermediates containing condensable polyhydroxycarbonoxy type radicals with a catalyst; and Weyenberg, U.S. Pat. No. 3,175,993, who combines organopolysiloxane intermediates end-blocked with alkoxylated silcarbonate groups with a catalyst.
Still other moisture curable one package RTV silicone compositions are disclosed in U.S. Pat. Nos. 3,689,454; 3,779,986; 3,294,739; 3,334,067; and 3,719,635.
Two other compositions of particular interest are disclosed by Beers in U.S. Pat. No. 4,100,129 and U.S. Pat. No. 4,257,932, respectively, each patent being assigned to the same assignee and the present invention. In U.S. Pat. No. 4,100,129 compositions are disclosed comprising: 100 parts by weight of a silanol chain-stopped polydiorganosiloxane; from 0.01 to 5.0 parts by weight of a cross-linking silane of the formula: EQU R'.sub.n Si(OR').sub.4-n
wherein R' and R" are each, independently, organic radicals of up to 8 carbon atoms selected from hydrocarbyl, halohydrocarbyl and cyano lower alkyl and n has a value of 0 to 3 and an average value based on the total amount of silane in the composition of 0 to 1.99; and from 0.10 to 10 parts by weight of a silanol reactive organometallic ester compound, wherein the weight of the organometallic ester compound to the cross-linking silane is always greater than 0.5 and preferably greater than 1.0. If compositions are prepared from the above-described ingredients in the specified ratios, a self-bonding, low modulus room temperature vulcanizing silicone composition is obtained which exhibits a very advantageous tensile and elongation relationship, e.g., low tensile and exceptionally high elongation, and in addition exhibits good adhesion to troublesome substrates.
In U.S. Pat. No. 4,257,932, an especially useful gasketing composition is disclosed which displays resistance to hot hydrocarbon oils and improved adhesion to substrates at elevated temperatures. This composition comprises a silanol chain-stopped polydiorganosiloxane, a fluid polysiloxane containing a high degree of tri- or tetrafunctionality or both, a finely divided silica filler, a polysilane crosslinker, and a crosslinking catalyst comprising an organic tin salt of an organic acid or a tin salt of an organic acid, with each ingredient being present in carefully specified amounts, and optionally containing a finely divided iron oxide thermal stabilizer.
Still another useful one package, room temperature vulcanizable silicone composition is described in copending application, Ser. No. 277,524, filed June 26, 1981 of White et al., now U.S. Pat. No. 4,395,526, assigned to the same assignee as the present invention. As described therein, especially stable room temperature vulcanizable organopolysiloxane compositions comprise:
(a) an organopolysiloxane wherein the silicon atom at each polymer chain end is terminated with at least two alkoxy radicals; PA0 (b) an effective amount of a condensation catalyst; PA0 (c) a stabilizing amount of a silane scavenger having the formula: ##STR2## wherein R' is a C.sub.1 -C.sub.8 aliphatic organic radical selected from the group consisting of alkyl, alkylether, alkylester, alkylketone, and alkylcyano radicals, or a C.sub.7 -C.sub.13 aralkyl radical; R" is a C.sub.1 -C.sub.13 monovalent substituted or unsubstituted hydrocarbon radical; X is a hydrolyzable leaving group selected from the group consisting of amido, amino, carbamato, enoxy, imidato, isocyanato, oximato, thioisocyanato and ureido radicals, and, a is an integer equal to 0 to 3 inclusive; where X is enoxy or amido, PA0 (d) an effective amount of a curing accelerator selected from the group consisting of substituted guanidines, amines and mixtures thereof; and PA0 (e) a cross-linking polyalkoxysilane of the formula: ##STR3## wherein R', R" and a are the same as R' and R" defined above. PA0 (a) 100 parts by weight of a silanol chain-stopped polydiorganosiloxane of the formula: ##STR8## wherein R.sup.6 and R.sup.7 are each, independently, organic radicals of up to 8 carbon atoms selected from hydrocarbyl, halohydrocarbyl and cyano lower alkyl and n is an average number of from about 10 to 15,000; PA0 (b) from 0.01 to 5.0, preferably, 0.1 to 0.95 parts by weight of a cross-linking silane of the formula: EQU R.sup.8.sub.m Si(OR.sup.9).sub.4-m PA0 wherein R.sup.8 and R.sup.9 are the same as defined for R.sup.6 and R.sup.7 hereinabove and m has a value of 0 to 3 and an average value based on the total amount of silane in the composition of 0 to 1.99; PA0 (c) from 0.1 to 10 parts by weight of a silanol reactive organometallic ester compound of a metal, the compound having radicals attached to the metal atom, at least one of said radicals being a hydrocarbonoxy radical or a substituted hydrocarbonoxy radical, said radicals being attached to the metal atoms through M--O--R linkages wherein M is the metal and any remaining valences of the metal are satisfied by substituents selected from organic radicals which are attached to the atom through M--O--C linkages, --OH and --O-- of a M--O--M linkage, the weight ratio of component (c) to (b) always being greater than 0.5 to 1, and preferably greater than unity; PA0 (d) from about 10 to 250 parts by weight of a filler per 100 parts of component (a) selected from the group consisting of calcium carbonate, fumed silica, and mixtures of the two; and PA0 (e) from about 0.2 to 2 parts by weight of an adhesion promoter composition per 100 parts of component (a), said adhesion promoting composition comprising at least one compound of the formula: ##STR9## wherein R.sup.1 may be C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 gamma-trihaloalkyl or phenyl; R.sup.2 is alkylene of from 2 to 6 carbon atoms; and R.sup.3, R.sup.4 and R.sup.5 are, independently, C.sub.1 -C.sub.6 alkyl or C.sub.2 -C.sub.6 alkanoyl and n is either 3 or 4. PA0 (a) 100 parts by weight of a silanol chain-stopped polydiorganosiloxane of the formula: ##STR10## wherein R.sup.6 and R.sup.7 are each, independently, organic radicals of up to 8 carbon atoms selected from hydrocarbyl, halohydrocarbyl and cyano lower alkyl and n is an average number of from 10 to 15,000; PA0 (b) from 2 to 20 parts by weight of a fluid polysiloxane containing a high degree of trifunctionality, tetrafunctionality or a mixture of tri- and tetrafunctionality and comprising: PA0 (C) from 10 to 100 parts by weight of a finely divided silica filler; PA0 (d) from 3 to 10 parts by weight of a cross-linking silane of the formula: EQU R.sup.10.sub.m Si(OR.sup.11).sub.4-m PA0 wherein R.sup.10 is defined the same as R.sup.6 and R.sup.7 above, and R.sup.11 is an organic radical of 6 to 30 carbon atoms selected from hydrocarboyl and halohydrocarboyl and m has a value of 0 to 1; PA0 (e) from 0.01 to 10 parts by weight of a curing catalyst selected from the group consisting of an organic tin salt of an organic acid, and a tin salt of an organic acid, said organic acid containing from 2 to 6 carbon atoms; and PA0 (f) from about 0.2 to 2 parts by weight of an adhesion promoter composition per 100 parts of component (a), said adhesion promoting composition comprising at least one compound of the formula: ##STR11## wherein R.sup.1 may be C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 gamma-trihaloalkyl or phenyl; R.sup.2 is divalent alkylene of from 2 to 6 carbon atoms; and R.sup.3, R.sup.4 and R.sup.5 are, independently, C.sub.1 -C.sub.6 alkyl or C.sub.2 -C.sub.6 alkanoyl and p is either 3 or 4. PA0 (a) 100 parts by weight of a polyalkoxy-terminated organopolysiloxane of the formula: ##STR12## wherein R.sup.14 is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, R.sup.12 is a C.sub.(1-8) aliphatic organic radical selected from the group consisting of alkyl, alkylether, alkylester, alkylketone and alkylcyano radicals, or a C.sub.(7-13) aralkyl radical, R.sup.13 is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, X is a hydrolyzable leaving group selected from the group consisting of amido, amino, carbamato, enoxy, imidato, isocyanato, oximato, thioisocyanato and ureido radicals and, b is a whole number equal to 0 or 1, e is a whole number equal to 0 or 1 inclusive and the sum of b+e is equal to 0 or 1 inclusive, and n is an integer having a value of from about 50 to about 2500 inclusive. PA0 (b) from about 0 to 10 parts by weight of a crosslinking silane of the formula: ##STR13## wherein R.sup.15 is a C.sub.(1-8) aliphatic organic radical selected from alkyl, alkylether, alkylester, alkylketone, alkylcyano or a C.sub.(7-13) aralkyl radical, R.sup.16 is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical and a is an integer equal to 0 or 1; PA0 (c) an effective amount of a condensation catalyst selected from the group consisting of tin compounds, zirconium compounds, titanium compounds, and mixtures thereof; PA0 (d) a stabilizing amount of a scavenging silane of the formula: ##STR14## wherein R.sup.17 and R.sup.18 are the same as R.sup.15 and R.sup.16 as defined in (b) above, X is a hydrolyzable leaving group selected from amido, amino, carbamato, enoxy, imidato, isocyanato, oximato, thioisocyanato, and ureido radicals, c is an integer equal to 0 to 3 inclusive, f is an integer equal to 1 to 4 inclusive, and the sum of c+f is equal to 1 to 4 inclusive; and PA0 (e) a minor effective adhesion promoting amount of an adhesion promoter composition comprising at least one compound of the formula: ##STR15## wherein R.sup.1 may be C.sub.1 -C.sub.6 alkyl, C.sub.1 -C.sub.6 gamma-trihaloalkyl or phenyl; R.sup.2 is alkylene of from 2 to 6 carbon atoms; R.sup.3, R.sup.4, and R.sup.5 are independently, C.sub.1 -C.sub.6 alkyl or C.sub.2 -C.sub.6 alkanoyl and n is either 3 or 4.
These useful polyalkoxy-terminated organopolysiloxane RTV compositions may also be prepared from compositions containing as a starting material a silanol chain-stopped diorganopolysiloxane together with a larger amount of the silane scavenger described as component (c) above, such that the silane scavenger acts both as a scavenger for hydroxy functional groups and a polyalkoxysilane cross-linking agent for terminating the silicon atom at each organopolysiloxane chain with at least two alkoxy radicals.
These compositions are described as stable, which means that each forms a moisture curable mixture capable of remaining substantially unchanged while excluded from atmospheric moisture and which cures to a tack-free elastomer after an extended shelf period. In addition, a stable RTV also means that the tack-free time exhibited by freshly mixed RTV ingredients under atmospheric conditions will be substantially the same as that exhibited by the same mixture of ingredients exposed to atmospheric moisture after having been held in a moisture resistant and moisture-free container for an extended shelf period at ambient conditions, or an equivalent period based on accelerated aging at an elevated temperature.
Although many of the prior art moisture-curable, one package room temperature vulcanizing compositions have been very useful in different applications, there is still room for improvement. For example, the benefits provided by these compositions and the areas of application could be extended if the adhesion of the compositions to various substrates at room temperature and other temperatures could be improved.
In the past, attempts to improve the adhesion of RTV silicone compositions to hard-to-bond substrates, such as metallic and thermoplastic surfaces, have generally fallen into three categories, i.e., treating the substrate with a primer material; using an adhesive composition to bond the silicone composition to the substrate; and lastly, incorporating adhesion-promoting additives in the RTV silicone composition itself. Of these three techniques, only the incorporation of additives provides a single step, single package product which is clearly the most demanded, useful and versatile.
In the prior art several different compounds have been added to RTV silicone compositions to improve their adhesion to substrates. For example, Kulpa, in U.S. Pat. No. 3,296,161 discloses that the addition of a dialkoxydiacyloxysilane having the formula (RO).sub.2 Si(OY).sub.2 wherein R is a lower alkyl radical and Y is a saturated aliphatic monoacyl radical of a carboxylic acid, to an RTV silicone composition comprising (a) a liquid organopolysiloxane containing from 0.02 to 2.0% by weight silicone bonded hydroxyl groups: (b) an organotriacyloxysilane, and (c) a carboxylic acid metal salt as a cure accelerator provides a product which exhibits improved adhesion to stainless steel and aluminum, as well as to surfaces such as glass and acrylic resins. In U.S. Pat. No. 3,438,930 to Beers it is disclosed that improved adhesion may result if a proper crosslinking silane is selected. More particularly, compositions comprising the reaction product of (a) a hydrolyzable silane of the formula: EQU R--Si--Y.sub.3
wherein Y is a hydrolyzable radical and R is selected from monovalent hydrocarbon radicals, halogenated monovalent hydrocarbon radicals, and R'.sub.3 CO radicals and R' is an alkyl radical and (b) an organopolysiloxane polymer terminated with a mixture of silanol radicals and tert-alkoxy radicals are described as exhibiting superior toughness and adhesion to a variety of unprimed substrates. Additionally, Hamilton et al. in U.S. Pat. No. 3,700,714, disclose that a acetodialkenyl substituted silanes are useful adhesion promoting crosslinking agents for silanol chain-stopped diorganopolysiloxanes. More particularly, Hamilton et al. disclose that silanes having one or more tert-alkoxy groups as silicone substituents, for example di-t-butoxy-diacetoxysilane, are effective adhesion promoters especially when bonding RTV compositions to substrates which are normally covered with an oxide film, such as aluminum.
Another useful family of adhesion promoting additives for room temperature vulcanizable silicone compositions is described by Berger in U.S. Pat. No. 3,517,001. The members of this family comprise the silicon-substituted isocyanuarates, known as mono-, di- and trisilylisocyanurates, of the formula: ##STR4## where R is an alkyl radical having from 1 to 8 carbon atoms, R' is a member selected from monovalent saturated hydrocarbon radicals, R" is selected from divalent saturated hydrocarbon radicals and halogenated divalent saturated hydrocarbon radicals, G is selected from R' radicals, (RO).sub.3-a --R'.sub.a Si R" radicals, aliphatically unsaturated monovalent hydrocarbon radicals, and halogenated hydrocarbon radicals and a is a whole number equal to 0 to 3, inclusive. Examples of preferred adhesion promoters from the above-described compounds are 1,3,5-tris-trimethyoxysilylpropylisocyanurate and bis-trimethoxysilylpropylisocyanurate.
Still another adhesion promoting additive useful in those applications where the RTV composition is to be bonded to a metal substrate is described in U.S. Pat. No. 3,719,635 to Clark et al. As disclosed therein, adhesion is promoted by the addition of from 0.01 to 2 parts by weight of a metal in the form of a metal hydrocarbonoxide selected from the group consisting of Fe(OR').sub.3, V(OR").sub.3, Co(OR").sub.2, MoO.sub.2 (OR").sub.2 ; Zn(OR").sub.2 ; Ce(OR").sub.3 and Al(OCH.sub.2 CH.sub.3).sub.3 wherein R' is selected from ethyl, normal propyl, normal butyl, and phenyl and R" is selected from ethyl, normal propyl and normal butyl.
Perhaps the most effective and widely used adhesion promoting additives, aside from the silylisocyanuarates disclosed by Berger above, are the carbamates and allophanates used and prepared in accordance with the teachings of Hardman et al., U.S. Pat. No. 4,306,813. As described therein useful adhesion promoters are provided in the form of a compound of the formula: ##STR5## wherein G is hydrogen or a group of the formula: ##STR6## and R is divalent radical selected from alkylenearylene, alkylene, cycloalkylene of up to 8 carbon atoms, R' and R"', independently, are radicals of up to 8 carbon atoms selected from hydrocarbyl or halohydrocarbyl, R" is a radical of up to 8 carbon atoms selected from hydrocarbyl, halohydrocarbyl or cyanohydrocarbyl and a is 0 to 3, or mixtures of such compounds. Although, these adhesion promoting additives have proved to be effective, they are very expensive to use. The need for a low cost, adhesion promoting additive for low modulus curable silicone compositions still exists.
Finally, mention should be made of glycidoxypropyltrimethoxysilane, which Beers discloses to be still another useful adhesion promoting additive for RTV silicone compositions in U.S. Pat. No. 4,257,932. Each of the above-cited patents and applications are specifically incorporated herein by reference.
It has presently been discovered that the adhesion of moisture curable one-package, low modulus, room temperature vulcanizable silicone compositions to various substrates is effectively improved by the addition of an adhesion promoter composition comprising at least one diorganocyclopolysiloxane. The adhesion promoters of the subject invention may be prepared and used at lower cost than the prior art isocyanurate adhesion promoters and their carbamate and allophanate derivatives, and generally provide at least as good adhesion to a variety of substrates.